Unit 2: Piston Engine Flashcards
Lecture 3, 4, 5 and 6. Only covers theory (a lot of maths in this unit to cover) (16 cards)
The main way a piston engine work?
A cylinder can have one or more pistons.
A hot and under pressure (steam engine) OR heated inside by ignition (internal combustion engine) OR hot heat exchange (Stirling engine) inside cylinder.
Hot gas expand - push piston to bottom of cylinder.
Piston return to top by flywheel or power of other piston connected by same shaft.
Most type the expanded/exhausted gases removed from cylinder by stroke (apart for Stirling as it heat and cool sealed gas)
What are the two fixed position of a cylinder?
The pistons reciprocates between the Top Dead Centre (TDC) - forming the smallest volume of cylinder and the Bottom Dead Centre (TDC) - forms largest volume in cylinder.
What does stroke, bore and the different valves mean?
The stroke is the largest distance the piston can travel in one direction (i.e. the TDC to BDC).
Bore is the diameter of piston.
The air fuel mixture drawn into cylinder by intake valve, and combustion product expelled by exhaust valve.
What do the terms clearance volume, displacement volume and compression ratio mean?
Clearance volume - minimum volume formed in cylinder when piston reach TDC.
Displacement Volume - volume displaced by piston between TDC and BDC.
Compression ratio (r) - ratio of the maximum volume formed in cylinder to the minimum (clearance volume).
What is the Mean Effective Pressure (MEP)?
A fictitious pressure that, if acted on the piston during the entire stroke, would produce the same amount of net work as that produced during an actual cycle.
What is MEP used for?
It can b used to compare the performance of reciprocating engine of equal size.
Higher Value will deliver more net work per cycle and thus better performance.
What are reciprocating engine classified as?
Spark-ignition (SI) engines or compression ignition (CI) engines.
- In SI engines, the combustion of the air-fuel mixture is initiated by a spark plug.
- In CI engines, the air-fuel mixture is self-ignited as a result of compressing the mixture above its self-ignition
temperature.
What are two stroke engine?
Two-stroke cycle are completed in two strokes. In two strokes the crankshaft makes one revolution, so the two-stroke cycle is complete in one revolution.
What does the Otto cycle consist of?
The cycle consists of four internally
reversible processes:
1-2 Isentropic compression
2-3 Constant volume heat addition
3-4 Isentropic expansion
4-1 Constant-volume heat rejection
What are the reason of loss in cycle?
- Valves take time to open and close, and combustion/exhaust
processes are not instantaneous (rounds off the corners of the
cycle) - Heat losses to sidewalls, valves etc
- Finite pumping work (shown as a negative work loop on the p-V
diagram.
What is the thermal efficiency of a heat engine
A heat engine is the
percentage of heat energy
that is transformed into work.
Methods to increase piston output?
- Evaporative cooling.
- Increasing engine speed.
- Increasing engine compression.
- Improving volumetric efficiency.
- Increasing number of cylinders.
How does propeller generally provide thrust?
Aerofoil sections designed to generate an aerodynamic force. The wing force provides lift to keep the airplane in the air, the propeller force provides thrust to push the airplane through the air.
Why are propeller more complex?
The propeller is analogous to a finite wing that has been twisted.
Therefore, the aerodynamics of the propeller are influenced by the same induced flow due to tip vortices as for a finite wing.
Why could propeller lose its power generated?
The kinetic energy is delivered by propeller, meaning it creates no useful work and robbing propellers available power.
- Slipstream
- Frictional loss (skin friction and pressure drag aka profile drag).
- Compressibility loss.
What is shaft power P?
- The power delivered to the propeller by the crank shaft.
- However, not all P is available
to drive the airplane; some of it is dissipated by inefficiencies of the
of the propeller itself.
